In the process of rotary drilling a well, a wellbore fluid or mud is circulated down the rotating drill pipe, through the bit, and up the annular space between the pipe and the formation or steel casing, to the surface. The wellbore fluid performs many different functions. For example, it removes cuttings from the bottom of the hole to the surface, suspends cuttings and weighting material when the circulation is interrupted, controls subsurface pressure, isolates the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, cools and lubricates the drill string and bit, maximizes penetration rate, etc. An important objective in drilling a well is also to secure the maximum amount of information about the type of formations being penetrated and the type of fluids or gases in the formation. This information is obtained by analyzing the cuttings and by electrical logging technology and by the use of various downhole logging techniques, including electrical measurements.
Various logging and imaging operations are performed during or after the drilling operation, for example, they may be performed while drilling in the reservoir region of an oil/gas well in order to determine the type of formation and the material therein. Such information may be used to optimally locate the pay zone, i.e., where the reservoir is perforated in order to allow the inflow of hydrocarbons into the wellbore. The use of wireline well logs is well known in the art of drilling subterranean wells and in particular oil and gas wells. A wireline log is generated by lowering a logging tool down the well on a wireline. The tool is slowly brought back to the surface and the instruments on the logging tool take measurements that characterize the formation penetrated by the well in addition to other important properties of the well. For example, during logging wireline logs may use measurements of relative resistivity of the formation to determine the geological composition of the downhole formation. An alternative or supplement to wireline logging involves logging tools placed in a specialized drill collar housing and run in the drill string near the bit. This technique is known as logging-while-drilling (LWD) or formation-evaluation-while-drilling (FEWD). Measurements such as electrical resistivity may be thereby taken and stored down-hole for later retrieval during a “tripping out” of the drill string, or transmitted to the surface via mud-pulse telemetry. Also, during drilling, such resistivity measurements may be useful to determine the location of the drill bit to enhance geosteering capabilities and directional drilling control, collected such as by a measurement while drilling (MWD) tool. Thus, electrical logs and other wireline log techniques are depended upon in the oil and gas exploration industry to determine the nature of the geology and the reservoir properties of the petroleum bearing formations penetrated by the well, as well as other properties of the drilling process (e.g., the location of the drill bit). Further, such well logs are often the only record of a formation penetrated by a well that are available for correlation amongst different wells in a particular field.